Earth continues to hit temperature and greenhouse gas milestones—just a couple of months ago, multiple stations measured carbon dioxide concentrations in the atmosphere of 400 parts per million, the highest in several million years. Many studies have tried to estimate how much and how rapidly the two great ice sheets covering Greenland and Antarctica might melt—and the one reassuring point has been the apparent relative stability of the eastern (and, by far, larger) half of the Antarctic Ice Sheet. Now, a new study of past melting in East Antarctica suggests that over the long haul, the "stable" ice sheet may be more vulnerable to warming than thought.

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To study possible future melting of the ice sheets, many scientists look to the past. Current warm temperatures and high greenhouse gas conditions are reminiscent of the warm Pliocene Epoch that lasted from 5.3 million to 2.6 million years ago. "Early and middle Pliocene global temperatures and greenhouse gas concentrations are probably the closest analog in Earth's history to the present climate on this planet and the climate conditions we will encounter before the end of this century," says Claus-Dieter Hillenbrand, a sedimentologist at the British Antarctic Survey in Cambridge, U.K., who was not involved in the study.

Abundant studies have examined melting of the Greenland Ice Sheet and the West Antarctic Ice Sheet (WAIS)—long considered the more vulnerable portion of the continent's ice, as much of it lies below sea level, flowing seamlessly into floating ice shelves. Satellite observations reveal that the WAIS is losing mass, sped up by melting along the base of the floating portions as the oceans warm. But the East Antarctic Ice Sheet (EAIS), which lies primarily on land, has appeared considerably more stable.

Yet, data from the Pliocene tell a different story, says Carys Cook, a doctoral student at Imperial College London. Mean temperatures from the Pliocene were 2°C to 3°C warmer than today, and atmospheric carbon dioxide concentrations between 350 and 450 parts per million. Some data have also suggested that sea levels were perhaps 22 meters higher than today—and even complete melting of the WAIS and Greenland couldn't account for more than about 12 meters of that, Cook says. Melting of the EAIS would have to have contributed.